This invention relates in general to magnetically actuated clutches and in particular to an improved structure for a magnetic flux breaker for minimizing the adverse effects of residual magnetism on the operation of a solenoid which controls the engagement and disengagement of a wrap spring clutch.
Clutches are well known devices which are frequently employed in machinery to selectively connect a source of rotational power to a rotatably driven mechanism. Typically, a clutch includes an input shaft, an output shaft, and some mechanism for selectively connecting the input shaft to the output shaft. When the clutch is engaged, the input shaft is connected to the output shaft so as to rotatably drive the driven mechanism. When the clutch is disengaged, the input shaft is disconnected from the output shaft.
One well known type of clutch is a wrap spring clutch. A basic wrap spring clutch includes an input hub or shaft, an output hub or shaft, and a coiled drive spring for selectively causing the input hub to rotatably drive the output hub. To accomplish this, the input hub and the output hub are provided with adjacent, axially aligned cylindrical surfaces. Portions of the drive spring are disposed about each of these cylindrical surfaces. The drive spring has a relaxed inner diameter which is slightly smaller than the outer diameter of the cylindrical surfaces of the input and output hubs. Thus, as is well known in the art, when the input hub is rotated in a first direction, the drive spring is wrapped tightly about the co-axially oriented cylindrical surfaces. As a result, the output hub is driven to rotate in the first direction with the input hub. When the input hub is rotated in a second direction, however, the drive spring is expanded about the co-axially oriented cylindrical surfaces. As a result, the output hub is not driven to rotate in the second direction with the input hub.
To control the engagement and disengagement of the wrap spring clutch efficiently, the drive spring is usually formed having a control tang at one or both ends thereof. The control tang is formed integrally with the drive spring and is provided for facilitating the expansion and contraction of the drive spring about the cylindrical surfaces of the input and output hubs. The control tang is fixed within a hollow cylindrical control collar disposed about the drive spring for rotation therewith. The external surface of the control collar is provided with one or more stops which are selectively engaged by a pivot arm. When the control collar is engaged by the pivot arm, the control tang is moved such that the drive spring expands about the input and output hubs, resulting in disengagement of the wrap spring clutch. When the control collar is not engaged by the pivot arm, the drive spring contracts about the input and output hubs, resulting in engagement of the wrap spring clutch. Movement of the pivot arm, therefore, determines whether the wrap spring clutch is engaged or disengaged.
A solenoid is usually connected to the pivot arm for moving it as described above to cause engagement and disengagement of the wrap spring clutch. As is also well known, the solenoid includes an armature which is axially movable in response to electrical current passed through an electromagnetic coil. When no electrical current is passed through the electromagnetic coil, the armature is urged toward a first position by a spring or other resilient return mechanism. When the electromagnetic coil is energized, the armature is drawn toward a second position against the urging of the spring. As a result, the pivot arm can be selectively moved into and out of engagement with the control collar to control the operation of the wrap spring clutch.
In some applications for wrap spring clutches, the speed of the machinery used in conjunction with the clutch is limited by the speed of movement of the armature when the electromagnetic coil is engaged. In order to increase the speed at which the armature is moved when the electromagnetic coil is energized, it is known to provide the solenoid with a core member formed from a magnetically permeable material. The core member is positioned axially adjacent to the electromagnetic coil and forms a focused path for magnetic flux generated by the energized electromagnetic coil. As a result, the intensity of the magnetic field generated by the energized electromagnetic coil is increased in the vicinity of the armature, and the armature is quickly moved when the electromagnetic coil is energized.
One impediment to rapid movement of the armature has been found to be residual magnetism created in the core member and armature. This residual magnetism is caused by the slight permanent magnetization of the core member and the armature by the magnetic field generated by the electromagnetic coil, especially when the coil is energized by a direct electrical current. The resultant slight magnetic attraction between the armature and the core member inhibits free relative movement therebetween, thus slowing the movement of the armature when the electromagnetic coil is deenergized. Also, because the armature repeatedly impacts the core member at high speeds, deformation of the inner axial end of the armature can occur, impairing or preventing operation of the solenoid.
To minimize the effects of residual magnetism and prevent the armature from impacting the core member, prior wrap spring clutch solenoids have provided articles referred to as flux breakers between the inner axial ends of the armatures and the adjacent core members. These prior art flux breakers have been embodied as relatively thin discs having outer diameters which are slightly less than the inner diameters of the associated electromagnetic coils. Prior art flux breakers have been formed from relatively soft, non-magnetically permeable materials, such as rubber, mylar, and bronze. By relatively soft, it is meant that the materials used to form the flux breaker were softer than the materials used to form the armature and the core member.
Flux breakers formed from these prior art materials having functioned satisfactorily for minimizing the effects of residual magnetism and limiting impact damage to the armature. Unfortunately, it has been found that such prior art flux breakers themselves were prone to deformation and damage as a result of repeated impacts by the armature. As a result, prior art flux breakers had to be replaced frequently, resulting in undesirable effort and expense for maintenance. Thus, it would be desirable to provide an improved structure for a magnetic flux breaker for minimizing the adverse effects of residual magnetism on the operation of a solenoid for controlling the engagement and disengagement of a wrap spring clutch which extends the useful life thereof.